Optical fiber amplifiers are commonly used in communication systems. Examples of optical fiber amplifiers include Erbium Doped Fiber Amplifiers (“EDFA”) and other type of Rare Earth Doped Fiber Amplifiers. The optical fiber amplifiers are usually pumped by one or more light emitter diodes (LEDs) or lasers. FIG. 1 illustrates a configuration for an EDFA 100 that is optically pumped by a light source, such as, a laser.
FIG. 1 illustrates a forward pumping scheme. With a forward pumping scheme, the optical signal to be amplified and the pumping light travel in a same direction. A first pump light Pl received from pump laser 102 is transmitted to a wave division multiplexing (WDM) coupler 108 as pumping light. An input optical signal Si, after passing through a tap 104 and an isolator 106 is coupled to WDM coupler 108. WDM coupler 108 combines the input optical signal Si and the pumping light Pl and provides an output to EDFA 100. Input optical signal Si is amplified in EDFA 100, and becomes output light So. Output light So is coupled to a tap 112 whose primary output is an amplified version of the input signal Si. The tap ports of tap 104 and tap 112 are coupled to detectors 110 and 114 whose outputs are coupled to a feedback circuit, e.g., PCB 120. Using the input power measured at detector 110 and the output power as detected at detector 114, the gain for EDFA 100 can be adjusted.
In the example shown in FIG. 1, two photodiodes (detectors 110 and 114) at respective input and output ports of EDFA 100 are used to detect input power Pin and output power Pout, respectively. The gain of EDFA 100 can be controlled by the pump power associated with the pumping light P1. More specifically, detector 110 measures the total input power Pin associated with the signal Si provided to the EDFA. Detector 114 detects the total output power Ptol of EDFA 100, which contains signal power Ps, amplified spontaneous emission (ASE) power Pase and pump residual power Ppr. In this example, signal gain is Pout/Pin=(Ptol−Pase−Ppr)/Pin. Conventional systems assume a correction factor for ASE power and pump residual power in order to produce accurate gain control, for example, in systems that require constant gain. If the wavelength of input signal is known, the correction factor can be determined accurately, thus the gain control is accurate. But if the wavelength of input signal is unknown, there exist problems in certain applications. More specifically different wavelength signals produce different ASE power and pump residual power for a given gain setting. When EDFA 100 is used in applications such as line-amplifiers and boosters, the output signal power, in general, is much larger than the ASE power and pump residual power. Accordingly, a near constant correction factor can be used to provide accurate gain control. But for EDFA applications such as preamplifiers with small input power (for example: −38 dBm) and smaller output power (for example: −14 dBm), the ASE power at the output port of the EDFA is larger than the output signal power. In these applications, the use of a constant correction factor will not produce accurate gain control. Further, the amount of pump residual signal Ppr depends on numerous factors including the power of the input signal, the wavelength of the input signal, and the performance of the amplifier and the WDM splitters. Again, a constant correction factor will not produce accurate gain control as the EDFA is used in different applications. What is desirable is an amplifier that automatically compensates for the pump residual and ASE in the output signal no matter the application.